US20170306436A1

US20170306436A1 - Steel sheet for two-piece can and manufacturing method therefor

Info

Publication number: US20170306436A1
Application number: US15/520,476
Authority: US
Inventors: Hayato Saito; Katsumi Kojima; Hiroki Nakamaru
Original assignee: JFE Steel Corp
Current assignee: JFE Steel Corp
Priority date: 2014-10-28
Filing date: 2015-09-18
Publication date: 2017-10-26
Also published as: KR20170063744A; MY178159A; JPWO2016067514A1; KR101989712B1; CN107002190A; WO2016067514A1; CN107002190B; JP6274302B2; TW201632636A; PH12017500433B1; TWI604067B; PH12017500433A1

Abstract

A steel sheet for two-piece cans is suitable not only for drawing and ironing, but also for forming beads or geometric shapes on a can body and can be preferably used to form a special-shaped two-piece can. The steel sheet for two-piece cans contains, in mass %, C: 0.020% to 0.080%, Si: 0.04% or less, Mn: 0.10% to 0.60%, P: 0.02% or less, S: 0.015% or less, Al: 0.010% to 0.100%, and N: 0.0005% to 0.0030%, the balance being Fe and unavoidable impurities. The steel sheet has a tensile strength of 480 MPa or more, an elongation of 7% or more, a yield elongation of 3% or less, and a ferrite grain size of less than 6 μm.

Description

TECHNICAL FIELD

This disclosure relates to a steel sheet for cans suitable for a can container material used for food cans and beverage cans and to a method of manufacturing the steel sheet. Particularly, the disclosure relates to a high-strength steel sheet for two-piece cans excellent in formability and to a method of manufacturing the high-strength steel sheet. The high-strength steel sheet for two-piece cans is preferably applicable to a special-shaped two-piece can having a processed can body.

BACKGROUND

From the viewpoint of environmental load reduction and cost reduction in recent years, there is a need for a reduction in the amount of use of steel sheets for food cans and beverage cans. Therefore, the thickness of steel sheets used as raw materials of two-piece cans and also three-piece cans is being reduced.
In recent years, to compensate for a reduction in strength of cans due to the reduction in thickness of the steel sheet, the cans are often formed as special-shaped cans produced by subjecting the can body to bead forming or by forming a geometric shape on the can body. When a special-shaped can composed of two pieces (which may be referred to as a special-shaped two-piece can) is manufactured, forming with a relatively high degree of working such as drawing or ironing is performed, and then the can body is formed. Therefore, the steel sheet used to manufacture the special-shaped two-piece can is required to have higher formability. In the can bottom subjected to a low degree of working, an increase in strength due to work hardening is small. Therefore, when a steel sheet reduced in thickness is used, the strength of the steel sheet at the can bottom tends to be insufficient. Particularly, in a negative pressure can having a flat can bottom, the steel sheet is required to have a strength equal to or higher than a conventional SR (Single Reduced) steel sheet. Therefore, it is effective to use a DR (Double Reduced) steel sheet for the can bottom portion because the DR steel sheet can be easily increased in strength even when it is reduced in thickness.
The DR steel sheet is hardened mainly by work hardening, and this generally causes its formability to deteriorate. Deterioration in formability is not preferred for the can body as described above. Therefore, techniques to improve formability of the DR steel sheet are being studied.
For example, Japanese Patent No. 3140929 discloses a resin-coated steel sheet for dry-drawn and ironed cans. The resin-coated steel sheet is prepared by coating both sides of an electrolytically chromic acid-treated steel sheet with a thermoplastic resin to a thickness of 10 to 50 μm and then applying a high-temperature volatile lubricant to the surface of the coating. The electrolytically chromic acid-treated steel sheet contains, in mass %, C: 0.001 to 0.10%, Mn: 0.05 to 0.50%, Al: 0.015 to 0.13%, Si: 0.05% or less, P: 0.03% or less, and S: 0.03% or less, the balance being Fe and unavoidable impurities. The electrolytically chromic acid-treated steel sheet has a crystal grain size of 6 to 30 μm, a center line average roughness of 0.05 to 0.6 μm, and a sheet thickness of 0.15 to 0.30 mm.
Japanese Patent No. 2937788 discloses a method of manufacturing a resin-coated steel sheet for dry-drawn and ironed cans. The method includes: pickling a hot-rolled sheet containing, in mass %, C: 0.001 to 0.06%, Mn: 0.05 to 0.50%, Al: 0.015 to 0.13%, Si: 0.05% or less, P: 0.03% or less, and S: 0.03% or less, the balance being Fe and unavoidable impurities; cold-rolling the pickled sheet; subjecting the cold-rolled sheet to continuous annealing; rolling the resultant sheet at a rolling reduction of 5 to 25% to obtain a rolled sheet having a center line average roughness of 0.05 to 0.6 μm and a thickness of 0.15 to 0.30 mm; subjecting the rolled sheet to electrolytic chromic acid treatment; coating both sides of the resulting rolled sheet with a thermoplastic resin to a thickness of 10 to 50 μm; and applying a high-temperature volatile lubricant to the surface of the coating.
Japanese Patent No. 4630268 discloses a steel sheet for special-shaped cans. The steel sheet has a steel composition containing, in mass %, C: 0.02 to 0.07%, Si: 0.005 to 0.05%, Mn: 0.1 to 1.5%, P: 0.04% or less, S: 0.02% or less, Al: 0.005 to 0.1%, N: more than 0.003 to 0.007%, and B: 0.001 to 0.01%, the balance being Fe and unavoidable impurities. In the steel sheet, the relation B/N: 0.3 to 1.5 is satisfied, and at least one of the Lankford value (r value) in the direction of rolling and the Lankford value (r value) in the direction of the sheet width is 0.8 or less.
The above conventional techniques, however, have the following problems.
With the technique described in JP '929, the formability necessary to form a straight can be ensured. However, with the technique described in JP '929, formability necessary to form a special-shaped can by subjecting the can body to processing such as bead forming cannot be ensured.
With the technique described in JP '788, as with the technique described in JP '929, formability necessary to form a straight can be ensured. However, also with the technique described in JP '788, formability necessary to form a special-shaped can cannot be ensured.
The technique described in JP '268 is for a three-piece can. In the steel sheet described in JP '268, at least one of the r value in the rolling direction and the r value in the sheet width direction is 0.8 or less and, therefore, the anisotropy of the steel sheet is large. The steel sheet with large anisotropy does not have the formability necessary for two-piece can forming including drawing.
It could therefore be helpful to provide a high-strength steel sheet for two-piece cans that can be particularly preferably used to form special-shaped two-piece cans and a method of manufacturing the high-strength steel sheet.

SUMMARY

We conducted extensive studies to find a method of simultaneously achieving the high strength necessary for a can bottom and the high formability necessary for a can body. Then we found that by controlling the chemical composition, tensile strength, elongation, yield elongation, and ferrite grain size of the steel sheet within specific ranges.
We thus provide:

- (1) A high-strength steel sheet for two-piece cans, comprising, in mass %, C: 0.020% to 0.080%, Si: 0.04% or less, Mn: 0.10% to 0.60%, P: 0.02% or less, S: 0.015% or less, Al: 0.010% to 0.100%, and N: 0.0005% to 0.0030%, the balance being Fe and unavoidable impurities, wherein the high-strength steel sheet has a tensile strength of 480 MPa or more, an elongation of 7% or more, a yield elongation of 3% or less, and a ferrite grain size of less than 6 μm.
- (2) The high-strength steel sheet for two-piece cans according to (1), further comprising, in mass %, B: 0.0001% to 0.0030%.
- (3) A method of manufacturing the high-strength steel sheet for two-piece cans according to (1) or (2), the method comprising: a heating step of heating a slab to a heating temperature of 1,130° C. or higher; a hot rolling step of hot rolling the slab subjected to the heating step under a condition of a hot rolling finishing temperature of 820 to 930° C. to obtain a hot-rolled sheet; a coiling step of coiling the hot-rolled sheet obtained in the hot rolling step at a coiling temperature of 640° C. or lower; a pickling step of pickling the hot-rolled sheet subjected to the coiling step; a primary cold rolling step of subjecting the pickled hot-rolled sheet to primary cold rolling under a condition of a rolling reduction of 85% or more to obtain a cold-rolled sheet; a continuous annealing step of subjecting the cold-rolled sheet obtained in the primary cold rolling step to continuous annealing under a condition of an annealing temperature of from 620° C. to 690° C. inclusive to obtain an annealed sheet; and a secondary cold rolling step of subjecting the annealed sheet obtained in the continuous annealing step to secondary cold rolling under a condition of a rolling reduction of 6 to 20%.

The high-strength steel sheet for two-piece cans has a specific controlled chemical composition. Moreover, the tensile strength of the high-strength steel sheet is controlled to 480 MPa or more, the elongation is controlled to 7% or more, the yield elongation is controlled to 3% or less, and the ferrite grain size is controlled to less than 6.0 μm. Therefore, the high-strength steel sheet for two-piece cans has the high strength necessary for a can bottom and also has the high formability necessary for a can body. With the high-strength steel sheet for two-piece cans, a special-shaped two-piece can be easily manufactured.
As described above, the steel sheet used to manufacture food cans and beverage can can be reduced in thickness, and resource savings and cost reduction can be achieved so that industrially significant effects can be obtained.

DETAILED DESCRIPTION

Examples will be described below. However, this disclosure is not limited to the Examples.

High-Strength Steel Sheet for Two-Piece Cans

The high-strength steel sheet for two-piece cans has a chemical composition containing, in mass %, C: 0.020% to 0.080%, Si: 0.04% or less, Mn: 0.10% to 0.60%, P: 0.02% or less, S: 0.015% or less, Al: 0.010% to 0.100%, and N: 0.0005% to 0.0030%, the balance being Fe and unavoidable impurities.
The high-strength steel sheet for two-piece cans has the following physical properties: a tensile strength of 480 MPa or more, an elongation of 7% or more, and a yield elongation of 3% or less.
The high-strength steel sheet for two-piece cans has a structure with a ferrite grain size of less than 6 μm.
The chemical composition, physical properties, and structure of the high-strength steel sheet for two-piece cans will be described in this order.
As described above, the high-strength steel sheet for two-piece cans contains, in mass %, C: 0.020% to 0.080%, Si: 0.04% or less, Mn: 0.10% to 0.60%, P: 0.02% or less, S: 0.015% or less, Al: 0.010% to 0.100%, and N: 0.0005% to 0.0030%, the balance being Fe and unavoidable impurities. The reasons that this chemical composition is used are as follows. In the following description, “%” representing the content of a component means “mass %.”

C: 0.020% to 0.080%

C is an element important to increase strength. When the content of C is 0.020% or more, the tensile strength can be 480 MPa or more. If the content of C exceeds 0.080%, elongation is reduced to less than 7% so that can manufacturability deteriorates. Therefore, the upper limit of the content of C must be 0.080%. As the content of C increases, the ferrite grain size decreases, and the strength increases. Therefore, preferably, the content of C is 0.030% or more. From the viewpoint of ensuring good canning property, it is preferable that the content of C is 0.060% or less.

Si: 0.04% or Less

When a large amount of Si is contained, surface treatability deteriorates due to surface enrichment, and this causes a reduction in corrosion resistance. Therefore, the content of Si must be 0.04% or less. Preferably, the content of Si is 0.03% or less.

Mn: 0.10% to 0.60%

Mn has the effect of increasing the hardness of the steel sheet through solid solution strengthening. Mn forms MnS, and this can prevent a reduction in hot ductility caused by S contained in the steel. To obtain these effects, the content of Mn must be 0.10% or more. Particularly, to ensure tensile strength through solid solution strengthening by Mn even when the rolling reduction during DR rolling is reduced, it is preferable that the content of Mn is 0.30% or more. If the content of Mn exceeds 0.60%, the elongation decreases significantly so that the canning property deteriorates. Therefore, the content of Mn must be 0.60% or less.

P: 0.02% or Less

If a large amount of P is contained, excessive hardening and center segregation occur, causing a reduction in formability. Moreover, if a large amount of P is contained, corrosion resistance deteriorates. Therefore, the upper limit of the content of P is 0.02%.

S: 0.015% or Less

S forms sulfide in the steel and causes deterioration in hot ductility. Therefore, the upper limit of the content of S is 0.015% or less.

Al: 0.010% to 0.100%

Al, together with N, forms AlN so that the amount of solute N in the steel is reduced. In this case, yield elongation is reduced, and stretcher strain is suppressed. Therefore, the content of Al must be 0.010% or more. From the viewpoint of reducing the yield elongation to thereby improve the canning property, the content of Al is preferably 0.050% or more and more preferably 0.060% or more. If the content of Al is excessively large, a large amount of alumina is formed. The formed alumina remains present in the steel sheet, causing deterioration in canning property. Therefore, the content of Al must be 0.100% or less. Preferably, the content of Al is 0.080% or less.

N: 0.0005% to 0.0030%

When N is present as solute N, the yield elongation increases, and stretcher strain occurs. In this case, surface appearance is poor and the canning property deteriorates. Therefore, the content of N must be 0.0030% or less. The content of N is preferably 0.0025% or less. It is difficult to reduce the content of N to less than 0.0005% in a stable manner. To achieve a N content of less than 0.0005%, the cost of manufacturing increases. Therefore, the lower limit of the content of N is 0.0005%.
Preferably, the high-strength steel sheet for two-piece cans contains, in addition to the essential components described above, B as an optional component in an amount of 0.0030% or less.

B: 0.0001 to 0.0030%

B, together with N, forms BN. In this case, the amount of solute N is reduced, and the yield elongation is reduced. Therefore, it is preferable that B is contained. To obtain the effect of the addition of B, the content of B is preferably 0.0001% or more and more preferably 0.0003% or more. Even if the content of B is excessively large, the above effect is saturated. In addition, the elongation is reduced, and anisotropy deteriorates so that the canning property deteriorates. Therefore, preferably, the upper limit of the content of B is 0.0030%.
The balance other than the above essential components and the optional component is Fe and unavoidable impurities. Examples of the unavoidable impurities include Cr: 0.08% or less, Cu: 0.02% or less, Ni: 0.02% or less, and O: 0.006% or less.
Next, the physical properties of the high-strength steel sheet for two-piece cans will be described. As described above, the high-strength steel sheet for two-piece cans has a tensile strength of 480 MPa or more, an elongation of 7% or more, and a yield elongation of 3% or less. The technological significance of each of these physical properties will be described below. However, one important technological significance is that by combining these physical properties, the chemical composition described above, and a structure described later, the high strength necessary for the can bottom and the high formability necessary for the can body can be achieved simultaneously.

Tensile Strength: 480 MPa or More

To ensure the strength of the can bottom, the tensile strength of the steel sheet must be 480 MPa or more. Preferably, the tensile strength is 490 MPa or more. A value obtained by a measurement method described in the Examples is used as the tensile strength of the steel sheet. The tensile strength is generally 580 MPa or less.

Elongation: 7% or More

To ensure formability of the can body during drawing, ironing, and also beading and the like, the elongation must be 7% or more. Preferably, the elongation is 9% or more. By setting the contents of the steel components within the prescribed ranges and using manufacturing conditions described later to reduce the ferrite grain size, a high strength of 480 MPa or more can be achieved, and also an elongation of 7% or more can be achieved so that the canning property can be ensured. A value obtained by the measurement method described in the Examples is used as the elongation of the steel sheet. Elongation is generally 25% or less.

Yield Elongation: 3% or Less

To prevent stretcher strain during can manufacturing, the yield elongation must be 3% or less. Preferably, the yield elongation is 2% or less. A value obtained by the measurement method described in the Examples is used as the yield elongation of the steel sheet.
Next, a description will be given of the structure of the high-strength steel sheet for two-piece cans. In the structure of the high-strength steel sheet for two-piece cans, the ferrite grain size is less than 6 μm.
Ferrite Grain Size: Less than 6 μm
By controlling the chemical composition of the steel sheet as described above and then reducing the ferrite grain size, the balance between strengthening and elongation is improved. Therefore, the ferrite grain size must be less than 6.0 μm. By reducing the ferrite grain size to less than 6.0 μm and reducing the yield elongation to 3% or less, the effect of improving adhesion between the surface of the steel sheet and a resin coating the steel sheet can also be obtained. From this point of view, it is preferable that the ferrite grain size is 5.5 μm or less. As will be described in the Examples, the grain size means the average crystal grain size.
The content of the ferrite phase in the structure is preferably 95 vol % or more because elongation can be improved. The content of the ferrite phase is more preferably 98 vol % or more. Examples of phases other than the ferrite phase include cementite, pearlite, martensite, and bainite.

Method of Manufacturing High-Strength Steel Sheet for Two-Piece Cans

One example of the method of manufacturing the high-strength steel sheet for two-piece cans is a manufacturing method including a heating step, a hot rolling step, a coiling step, a pickling step, a primary cold rolling step, a continuous annealing step, and a secondary cold rolling step. These steps will next be described.

Heating Step

The heating step heats a slab to a heating temperature of 1,130° C. or higher. If the heating temperature before hot rolling is excessively low, part of AlN is not molten. The non-molten AlN may cause the occurrence of coarse AlN that reduces the canning property. Therefore, the heating temperature in the heating step is 1,130° C. or higher. Preferably, the heating temperature is 1,150° C. or higher. The upper limit of the heating temperature is not particularly specified. However, if the heating temperature is excessively high, an excessive amount of scales is formed, resulting in defects on the surface of the product. Therefore, preferably, the upper limit of the heating temperature is 1,260° C.
The chemical composition of the slab corresponds to the chemical composition of the high-strength steel sheet for two-piece cans. Therefore, the chemical composition of the slab must be controlled to satisfy the chemical composition of the high-strength steel sheet for two-piece cans.

Hot Rolling Step

The hot rolling step is the step of hot rolling the slab subjected to the heating step at a hot rolling finishing temperature of 820 to 930° C. If the hot rolling finishing temperature is higher than 930° C., the ferrite grain size of the hot-rolled sheet becomes large, and the ferrite grain size of an annealed sheet also becomes large. In this case, the tensile strength decreases, and the tensile strength and the elongation are not well-balanced. Therefore, the upper limit of the hot rolling finishing temperature is set to 930° C. If the hot rolling finishing temperature is lower than 820° C., the anisotropy of tensile characteristics becomes large, and the canning property deteriorates. Therefore, the lower limit of the hot rolling finishing temperature is 820° C. The lower limit is preferably 860° C.

Coiling Step

The coiling step is the step of coiling the hot-rolled sheet obtained in the hot rolling step at a coiling temperature of 640° C. or lower. If the coiling temperature exceeds 640° C., the ferrite grain size of the hot-rolled sheet becomes large, and the ferrite grain size of the annealed sheet also becomes large. In this case, the tensile strength decreases, and the tensile strength and the elongation are not well-balanced. Therefore, the upper limit of the coiling temperature is set to 640° C. The lower limit of the coiling temperature is not particularly specified. From the viewpoint of reducing the yield elongation by forming AlN during coiling to reduce the amount of solute N, it is preferable that the coiling temperature is set to 570° C. or higher.

Pickling Step

The pickling step pickles the hot-rolled sheet subjected to the coiling step. The conditions of pickling are not particularly specified so long as surface scales can be removed. The pickling may be performed according to a routine procedure.

Primary Cold Rolling Step

The primary cold rolling step subjects the pickled hot-rolled sheet to primary cold rolling under the condition of a rolling reduction of 85% or more. The rolling reduction during the primary cold rolling must be 85% or more to reduce the ferrite grain size after annealing to thereby improve the balance between the tensile strength and formability. If the rolling reduction during the primary cold rolling is excessively large, the anisotropy of the tensile characteristics becomes large, and the canning property may deteriorate. Therefore, preferably, the rolling reduction during the primary cold rolling is 90% or less.

Continuous Annealing Step

The continuous annealing step subjects the cold-rolled sheet obtained in the primary cold rolling step to continuous annealing under the condition of an annealing temperature of 620° C. to 690° C. To ensure formability, the cold-rolled sheet must be recrystallized sufficiently during annealing. Therefore, the annealing temperature must be 620° C. or higher. If the annealing temperature is excessively high, the ferrite grain size becomes large. Therefore, the annealing temperature must be 690° C. or lower. No limitation is imposed on the annealing method. From the viewpoint of the uniformity of material quality, a continuous annealing method is preferred.

Secondary Cold Rolling Step

The secondary cold rolling step subjects the annealed sheet obtained in the continuous annealing step to secondary cold rolling under the condition of a rolling reduction of 6 to 20%. The annealed sheet is strengthened by the secondary cold rolling and is also reduced in thickness. To sufficiently achieve the strengthening, the rolling reduction must be 6% or more. As a result of the secondary cold rolling, the yield elongation decreases. If the rolling reduction during the secondary cold rolling is excessively large, formability deteriorates. Therefore, the rolling reduction must be 20% or less. When formability is particularly required, it is preferable that the rolling reduction is 15% or less.
The high-strength steel sheet for two-piece cans is obtained in the manner described above. The steel sheet may be subjected to surface treatment such as Sn plating, Ni plating, or Cr plating, may be subjected to chemical conversion, or may have an organic coating such as a laminate.

Examples

Molten steel containing components in one of steel symbols A to K shown in Table 1 with the balance being Fe and unavoidable impurities was produced, and then a steel slab was obtained. The steel slab obtained was heated, hot-rolled, coiled, pickled to remove scales, subjected to primary cold rolling, and annealed for 15 s in a continuous annealing furnace under the conditions shown in Table 2 and was then subjected to DR rolling (secondary cold rolling) at a secondary rolling reduction shown in Table 2 to thereby obtain a steel sheet (one of steel sheet symbols Nos. 1 to 18) having a sheet thickness of 0.17 to 0.19 mm. Each of the above steel sheets was subjected to surface treatment, i.e., (tin-free) chromium plating, and was coated with an organic coating to produce a laminated steel sheet.

TABLE 1

Steel
symbol	C	Si	Mn	P	S	Al	N	B	Remarks

A	0.045	0.01	0.52	0.013	0.011	0.061	0.0025	—	Example
B	0.080	0.02	0.14	0.011	0.009	0.073	0.0024	—	Example
C	0.060	0.01	0.51	0.018	0.012	0.055	0.0020	—	Example
D	0.046	0.01	0.60	0.008	0.008	0.093	0.0030	—	Example
E	0.020	0.01	0.33	0.016	0.011	0.085	0.0016	—	Example
F	0.038	0.01	0.36	0.014	0.013	0.063	0.0026	0.0007	Example
G	0.046	0.02	0.55	0.010	0.011	0.050	0.0025	0.0026	Example
H	0.121	0.01	0.24	0.014	0.012	0.043	0.0019	—	Comparative Example
I	0.011	0.01	0.34	0.012	0.009	0.061	0.0025	—	Comparative Example
J	0.055	0.01	0.83	0.013	0.010	0.046	0.0023	—	Comparative Example
K	0.049	0.01	0.04	0.016	0.007	0.070	0.0027	—	Comparative Example

TABLE 2

					Primary cold		Secondary
Steel		Heating	Finishing	Coiling	rolling	Annealing	rolling	Sheet
sheet	Steel	Temperature	temperature	temperature	reduction	temperature	reduction	thickness
symbol	symbol	° C.	° C.	° C.	%	° C.	%	mm	Remarks

No1	A	1160	880	620	89.9	660	10	0.190	Example
No2	B	1200	890	610	89.7	680	12	0.190	Example
No3	C	1180	860	570	90.0	620	10	0.180	Example
No4	D	1160	880	640	89.1	650	8	0.180	Example
No5	E	1170	870	620	88.9	690	15	0.170	Example
No6	F	1180	910	590	88.0	650	12	0.190	Example
No7	G	1210	900	600	89.2	670	12	0.190	Example
No8	H	1220	880	630	89.2	660	12	0.190	Comparative Example
No9	I	1160	860	600	89.4	670	10	0.190	Comparative Example
No10	J	1160	870	620	89.2	670	12	0.190	Comparative Example
No11	K	1160	880	640	89.8	660	12	0.180	Comparative Example
No12	A	1160	880	600	89.2	600	12	0.190	Comparative Example
No13	A	1160	870	610	89.4	730	10	0.190	Comparative Example
No14	A	1170	870	620	90.0	670	5	0.190	Comparative Example
No15	A	1170	890	600	87.3	660	25	0.190	Comparative Example
No16	A	1160	870	550	89.4	670	10	0.190	Example
No17	A	1150	880	700	89.2	670	12	0.190	Comparative Example
No18	A	1160	880	580	90.0	660	6	0.180	Example

Tensile Strength, Elongation, and Yield Elongation

The organic coating was removed from each of the laminated steel sheets using concentrated sulfuric acid, and a JIS No. 5 tensile test piece was taken from each laminated steel sheet in its rolling direction. The tensile strength, elongation (total elongation), and yield elongation of the test piece were evaluated according to JIS Z 2241.

Ferrite Grain Size

A cross section in the rolling direction was embedded, polished, and etched with nital to allow grain boundaries to appear. Then the average crystal grain size was measured using a cutting method according to JIS G 0551 to evaluate the ferrite grain size.
Evaluation of Property Forming into a can
To evaluate the property forming into a can, one of the laminated steel sheets was punched into a circular piece, and the circular piece was subjected to deep drawing, ironing or the like to obtain a cylindrical can. Then, the cylindrical can was subjected to beading in the circumferential direction of the can at three different places including the height center of the can body and places 15 mm vertically from the height center to thereby form a can similar to a two-piece can used as a beverage can. A can with no can body breakage during can manufacturing and with almost no stretcher strain was rated “Excellent.” A can with no can body breakage but with slight stretcher strain was rated “Good.” A can with can body breakage or significant stretcher strain was rated “Poor.”
The results are shown in Table 3. In all Examples, the tensile strength was 480 MPa or more, the elongation was 7% or more, the yield elongation was 3% or less, and the ferrite grain size was less than 6.0 μm. Therefore, each of the steel sheets has excellent formability and strength. However, in Comparative Examples, at least one of the above properties was poor. For example, in steel sheet symbols Nos. 9, 11, 13, and 17, although the canning property rating was “Good,” the tensile strength of the steel sheet was low and was not sufficient for the can bottom.

TABLE 3

						Evaluation
Steel		Tensile		Yield	Ferrite	of
sheet	Steel	strength	Elongation	Elongation	grain size	canning
symbol	symbol	Mpa	%	%	μm	property	Remarks

No1	A	505	9	1.2	5.4	Excellent	Example
No2	B	540	8	2.3	5.1	Good	Example
No3	C	500	10	1.4	5.3	Excellent	Example
No4	D	529	11	1.3	4.8	Excellent	Example
No5	E	540	7	0.8	5.8	Excellent	Example
No6	F	520	9	0.4	5.5	Excellent	Example
No7	G	527	7	0.2	5.2	Excellent	Example
No8	H	595	5	6.1	5.2	Poor	Comparative Example
No9	I	458	10	1.0	7.1	Good	Comparative Example
No10	J	580	4	1.6	5.6	Poor	Comparative Example
No11	K	450	9	2.1	7.3	Good	Comparative Example
No12	A	590	3	0.2	5.0	Poor	Comparative Example
No13	A	450	12	2.8	8.1	Good	Comparative Example
No14	A	432	20	4.2	5.5	Poor	Comparative Example
No15	A	580	2	0.0	5.3	Poor	Comparative Example
No16	A	510	8	2.9	5.5	Good	Example
No17	A	460	7	2.2	6.3	Good	Comparative Example
No18	A	480	13	2.6	5.5	Good	Example

Claims

1-3. (canceled)

4. A steel sheet for two-piece cans, comprising, in mass %, C: 0.020% to 0.080%, Si: 0.04% or less, Mn: 0.10% to 0.60%, P: 0.02% or less, S: 0.015% or less, Al: 0.010% to 0.100%, and N: 0.0005% to 0.0030%, the balance being Fe and unavoidable impurities,

wherein the high-strength steel sheet has a tensile strength of 480 MPa or more,

an elongation of 7% or more,

a yield elongation of 3% or less, and

a ferrite grain size of less than 6 μm.

5. The steel sheet for two-piece cans according to claim 4, further comprising, in mass %, B: 0.0001% to 0.0030%.

6. A method of manufacturing the steel sheet for two-piece cans according to claim 4 comprising:

heating a slab to a heating temperature of 1,130° C. or higher;

hot rolling the slab subjected to the heating under a condition of a hot rolling finishing temperature of 820 to 930° C. to obtain a hot-rolled sheet;

coiling the hot-rolled sheet obtained in the hot rolling at a coiling temperature of 640° C. or lower;

pickling the hot-rolled sheet subjected to the coiling;

subjecting the pickled hot-rolled sheet to primary cold rolling under a condition of a rolling reduction of 85% or more to obtain a cold-rolled sheet;

subjecting the cold-rolled sheet obtained in the primary cold rolling to continuous annealing under an annealing temperature of 620° C. to 690° C. to obtain an annealed sheet; and

subjecting the annealed sheet obtained in the continuous annealing to secondary cold rolling at a rolling reduction of 6 to 20%.

7. A method of manufacturing the steel sheet for two-piece cans according to claim 5 comprising:

heating a slab to a heating temperature of 1,130° C. or higher;

pickling the hot-rolled sheet subjected to the coiling;